An electric actuator is a device that is capable of converting electrical energy into mechanical energy, for example in form of linear or rotational motions, to drive other devices. The electric actuator can be designed and fabricated in a miniature size on a semiconductor substrate by micromachining and/or semiconductor processing technology. Such electric actuator is a type of micro-electro-mechanical system (MEMS) that can augment microelectronics, such as a system on a chip (SOC), by generating motions in order to control its environment in response to signals received from the microelectronics.
FIG. 1 illustrates a schematic top view of a typical MEMS actuator 10 comprising a stator 12, a mover 14 capable of moving relative to the stator 12, and a number of return springs 20, 22, and 24. The stator 12 can have a plurality of first electrodes 16 extending therefrom toward the mover 14. The mover 14 can have a plurality of second electrode 18 extending therefrom toward the stator 12. At least one of the first electrodes 16 can be disposed between two consecutive second electrodes 18 in a manner where the first electrode 16 is closer to one of the second electrodes 18 than the other. Likewise, at least one of the second electrodes 18 can be disposed between two consecutive first electrodes 16 in a manner where the second electrode 18 is closer to one of the first electrodes 16 than the other. The mover 14 can have one or more protrusions 19 extending from one or more sides of its body, respectively. The return springs 20 and 22 can be coupled between the mover 14 and their respective fixtures 26 and 28. Each of the return springs 24 can have one end coupled to a fixture 30, and another end disposed in proximity of its corresponding protrusion 19.
In operation, the first and second electrodes 16 and 18 can be electrically charged to create a capacitive force between them. As discussed above, because the first and second electrodes 16 and 18 are interposed in an asymmetric manner, the capacitive force generated can move the mover 14 in a desired direction relative to the stator 12 as shown by an arrow 13 in the figure. As the mover 14 moves away from its initial position, the return springs 20, 22, and 24 may deflect to provide it with a return force that is necessary to push the mover 14 back to its initial position after the first and second electrodes 16 and 18 become discharged. The actuator 10 generates motions by closing the gaps between the first and second electrodes 16 and 18. Thus, the actuator 10 is typically named as a gap-closing actuator.
One of the shortcomings of the conventional gap-closing actuator 10 is the difficulty in controlling its fabrication process. In fabrication, the first and second electrodes 16 and 18 are typically formed by performing an etching process on a semiconductor substrate. As shown in FIG. 1, the first and second electrodes 16 and 18 are interposed among each other in an asymmetric manner where a gap D1 between at least one of the first electrodes 16 and one of two consecutive second electrodes 18 between which the first electrode 16 is disposed is wider than a gap D2 between the first electrode 16 and the other of the two consecutive second electrodes 18. Due to different aspect ratios of the gaps D1 and D2, the etch rate of the semiconductor material in the wider gap D1 can be faster than that in the narrower gap D2. Thus, it is difficult to fabricate the wider gap D1 and the narrower gap D2 in the same depth, which in turn makes it difficult to control the size of the actuator 10 accurately.
Moreover, the smaller the line width of the electrodes and the larger the ratio D1 to D2, the more difficult it is to control the etch rate variation in fabrication. Thus, in order to control the etch rate variation in a manageable range, the minimum width of the gap D2 that can be selected for the actuator 10 is limited. This, in turn, leads to greater power consumption of the actuator, because the wider the gap D2, the higher the voltage it is required to operate the actuator 10 for a given return spring constant.
Another shortcoming of the conventional gap-closing actuator 10 is that the asymmetric arrangement of electrodes 16 and 18 prevents the size of the actuator 10 from being reduced beyond limits imposed by lithographical processes. In fabrication, the gaps D1 and D2 among the electrodes 16 and 18 are defined by a photo mask during a lithographic process, which typically has a limit for a minimal line width that can be defined. Although the narrower gap D2 can be defined as being equal to the minimal line width of the lithographic process, the wider gap D1 is certainly broader than the minimal line width because it needs to be longer than the narrower gap D2 in order to ensure that the mover 14 moves in a desired direction when the actuator 10 is in operation.
Accordingly, what is needed is a gap-closing actuator whose fabrication process can be accurately controlled, power consumption can be decreased, and size can be reduced beyond limits imposed by lithographic processes.